I coat my paper for gum in a 3 % gelatin solution by immersion in a tray.
After drying (at least one night) I soak the paper in 1 liter of
water containing 25 ml of a 40% glyoxal solution with some borax
added. In fact I am copying exactly as I was doing with formalin.

After 2-3 minutes in the bath I put the sheets in a tray with running
water and leave them in for some minutes.

After drying there is no apparent yellowing. But....when these papers
are kept (in the dark) for some weeks an significant yellowing is
visible alover but most apparent at the edge where the gelatin is a
litlle thicker.

The yellowing was more severe when I started using glyoxal. At that
time I did not rinse the paper afterwards as I do now.

I think there is indeed a relation between concentration and yellowing.

glutaraldehyde

Apart from this I looked at glutaraldehyde too and found in the
archives that MACO hardener (LP-Geladur and MACO LPE 510 hardening
additive for Black Magic emulsion) contains glutaraldehyde too.

As I have a bottle of LP-Geladur here,I asked MACO for the exact
proportions and they kindly did send me a recent (Feb.9 2004) safety
sheet. It says:

disodium disulphite 5-10 %
glutaraldehyde 2-5 %
acetic acid 0,5-2 %

Apart from these figures being not too exact (but good enough for
safetysheets I presume) one can see indeed that the glutaraldehyde
concentration is rather low. Especially when knowing that the
solution is a concentrate to be diluted before adding (to the liquid
emulsion). For hardening their photogelatin they advise a working
solution of 5 ml concentrate on a total of 100 ml water. From this
dilution one should add 20-50 ml per liter gelatin solution.

What I would like to know (I will try if it works myself too of
course) what the acid and the bisulphe are doing in the hardening
process. Disulphite is often advised to neutralise glutaraldehyde
before disposal. And I was thinking that an acid environment
prevented an effective hardening by aldehydes.

Kees Brandenburg

>On Sun, 15 Feb 2004, Christina Z. Anderson wrote:
>
>> However, one observation I'll have to corroborate the next time I size: the
>> glyoxal gelatin solution seemed yellower today than last night. Whether
>> this leads to any explanation of the yellowing controversy we have discussed
>> here, who knows. It was kept at 140-120 degrees for 15 hrs and is yellower.
>> Any suggestions, chemists and gum printers? When brushed on the paper, the
>> yellowing in a thin coat of size is not apparent so no biggie. Plus I'll be
>> using it right away so it won't sit in a drawer to get yellower...
>
>
>Of course I repeat myself, but my suggestion is to apply the hardener as
>a separate coat -- you save all kinds of variables that occur when it's in
>the gelatin, including the aging and hardening of the combo... (Of course
>on the other hand, who knows what interesting creatures you could grow in
>a gelatin culture kept at 120 degrees for a day or two...)
>
>I also suspect that you'd use less glyoxal, which is a lot more expensive
>than gelatin... That is, unless you're brush coating the gelatin too...?
>
>For a separate hardening bath, I use 15 ml glyoxal per litre of water (the
>same amount I used for formaldehyde)... the sheets can be hardened a few
>at a time so a shallow tray full is OK... I rinse the paper after
>hardening -- and do not see any yellowing.
>
>J.
Received on Mon Feb 16 04:16:18 2004